Combined experimental flow visualization and three-dimensional unsteady numerical simulation were carried out to investigate the formation of the traveling transverse and mixed vortex roll structures from the initially unidirectional, subcritical flow of air in a bottom heated horizontal flat duct when the Rayleigh number was raised to a supercritical level. In the study the Reynolds number was varied from 2.5 to 20 and the Rayleigh number from 1500 to 8000. Both the experiment and computation, being in qualitative agreement, indicated that in the early stage of the transient a pair of longitudinal rolls are induced in the side wall region; then new longitudinal rolls were induced near the existing ones. Meanwhile, weak transverse rolls were generated in the duct entry. As time proceeds, the transverse rolls grow in size and strength and slowly move downstream to push the longitudinal rolls in front of them out of the duct. At the same time new transverse rolls are generated in the duct entry. Ar a low Reynolds number (Rr less than or equal to 5) the transverse rolls are longer and stronger to expel all the longitudinal rolls out of the test section so that a pure and regular transverse vortex roll structure is formed. While at a higher Reynolds number (Re greater than or equal to 7.5) the transverse rolls are shorter and weaker and a few longitudinal rolls stay, all the time, near the duct sides, resulting in a mixed longitudinal and transverse roll structure. In the numerical simulation the temporal characteristics of the two vortex flow structures at the statistical state are also examined in detail.